In the case of reversible rolling on a product strip by use of a reversible rolling mill, the rolling is performed with the rolling mill (mill proper body), equipped with an unwinder (pay-off reel) disposed on the entry side, and a winder disposed on each of the entry side and the delivery side. The front end portion of the strip is wound and held on the delivery side winder by two or three turns made at the beginning of the first pass rolling, and then the rear end portion of the strip is wound and held on the entry side winder by two or three turns made at the beginning of the second pass rolling, so that the rolling is performed while tension is given to the strip. Then, the rolling is performed by a plurality of passes made back and forth while the front end portion and rear end portion are kept held on the winders. After the strip is processed to a predetermined thickness by a predetermined number of passes of rolling, the strip is unloaded as a coil from one of the winders. The strip includes parts not to be product, which are off-gauge parts, at the front end portion and rear end portion because they are not rolled, and so these parts are cut off and discarded immediately before unloading or in the subsequent step.
The off-gauge parts generated at both end portions of the strip in the longitudinal direction bring about a loss in the strip as described above, and so they are preferably to be set as short as possible in light of the yield ratio. Patent Document 1 listed below discloses a metal rolling method arranged to reduce such a loss and thereby improve the yield ratio.
According to the method disclosed in Patent Document 1, a leader strip is held by the winder on the delivery side or on the entry side, and is welded to the front end portion or rear end portion of a product strip, so that the product strip is rolled by the rolling mill nearly up to the end portion thereof while tension is given from the winder through the leader strip. Specifically, as shown in FIG. 12, (a) to (f), a product strip A is wound off from a coil (hot coil) CR1 attached to the unwinder 2, and is subjected to reversible rolling between a rolling mill 1 and winders 3 and 4, in accordance with the following steps.
a) At first, the front end portion (delivery side end portion) of the product strip A is wound off from the unwinder 2. When the front end portion reaches the position of a welder 12′, this front end portion and an end portion of a leader strip Y, which has been wound off from the delivery side winder 4, are overlapped with each other and spot-welded to each other by the welder 12′ (FIG. 12, (a)).
b) Then, the welded part Yc formed by the spot-welding is unwound to a position near the rolling mill 1, and the rolls of the rolling mill 1 are closed to start the first pass rolling (FIG. 12, (b)).
c) The rolling is performed while tension is given to the product strip A by the winder 4 and the other winders. When the rear end portion of the product strip is separated from the unwinder 2, the strip press 15′ is used to give tension to the product strip A (FIG. 12, (c)).
d) When the rear end portion (entry side end portion) of the product strip A reaches the position of the welder 11′, a leader strip X is unwound from the entry side winder 3. The rear end portion of the product strip A and the leader strip X are overlapped with each other at the welder 11′ and welded to each other by the welder 11′ (FIG. 12, (d)).
e) Then, the rolling is performed while tension is given by the entry side winder 3 until the welded part Xc reaches a position near the rolling mill 1, thereby finishing the first pass rolling. Then, the second pass rolling and the subsequent even ordinal number passes of rolling are performed while tension is given by the delivery side winder 4 until the welded part Yc between the product strip A and leader strip Y reaches a position near the rolling mill 1 (FIG. 12, (e)).
f) The third pass rolling and subsequent odd ordinal number passes of rolling are performed while tension is given to the product strip A, as described above, until the welded part Xc reaches a position near the rolling mill 1 again (FIG. 12, (f)).
As described above, odd ordinal number passes of rolling and even ordinal number passes of rolling are repeated to perform reverse rolling until the product strip A is processed to a predetermined thickness. After the rolling is completed, the coil of the product strip is unloaded either from the entry side or delivery side winder. In FIG. 12, reference symbols 13′ and 14′ denote cutters.
Where the metal rolling is performed as described above, since the leader strips are welded to the end portions of the product strip in the longitudinal direction (i.e., portions hard to roll with tension given thereto), most of the product strip is subjected to the rolling, whereby the loss is reduced. Further, since the rolling is performed while tension is given from the winders through the leader strips, other advantages, such that the feed of the product strip becomes stable without meanderings, and the product is formed accurately in shape and thickness, are brought about.
[Patent Document 1]    Jpn. Pat. Appln. KOKAI Publication No. 2006-334647
Patent Document 1 mentioned above discloses a method for reducing the off-gauge part loss in strip metal rolling performed by use of a reversible rolling mill, thereby improving the yield ratio, but this method cannot provide the most advantageous measure in light of the productivity as well. Where the leader strips are attached to both of the front end portion and rear end portion, the yield ratio is remarkably improved, but, on the other hand, it takes time to treat the leader strips left inside the coil, thereby deteriorating the productivity. In other words, conventionally, no optimum sequence for maximizing the productivities has been found in association with the welding and subsequent separation of leader strips. Further, for example, no specific welder or optimized welder has been provided to weld a leader strip to a product strip.